1. Field of the Invention
The present invention relates to a glass fiber bushing and more particularly to a hermetically sealed bushing for feeding a glass fiber through a metal part in which the fiber is fixed in a hole by soldering.
2. Description of the Prior Art
Such fiber bushings are used, for example, in optical communications, in particular for hermetically sealed housings of electro-optical or opto-electrical components which must be protected against environmental influences.
To fix the bare glass fiber, a metal solder is preferably used, which has a higher temperature resistance than adhesive and is moisture proof. The solder prevents the glass fiber from "creeping".
Printed publications DE-OS No. 27 21 991 and DE-OS No. 30 46 415 teach that it is necessary to metallize the glass fiber prior to soldering. The metallization must meet several quality requirements. On the one hand, it must adhere very well to the glass fiber. On the other hand, the metallization must be easily solderable.
To achieve good adhesion, it is essential that the fiber surface be very clean. Cleaning may be accomplished using one of several alternative process steps, e.g., by immersing the glass fiber in hot sulfuric acid with subsequent washing in deionized water and drying, or by chemically etching the glass fiber in hydrofluoric acid with subsequent washing and drying.
However, since firmly adhering metallization coatings can often be soldered only with difficulty, or not at all, two or more metal coatings must be applied either successively, or simultaneously as a mixture. This can be effected by evaporation or sputtering in a vacuum, or by a wet chemical process. To this end, tools, devices and means are required which are available in glass fiber bushing manufacturing facilities, but not in the fleld. Accordingly, glass fibers cannot be practically metallized, and, consequently, not soldered at the sites where optical transmission devices are being installed and where the required locations of bushings can be determined.
Even during manufacturing, difficulties occur when glass fibers are metallized since the fiber can easily break. Fibers are particularly vulnerable to breakage at the interface between the treated and untreated surfaces, or at the transition to the metallization coating. Moreover, it has been found that the metallization tends to dissolve during soldering, the adhesion thus being reduced or lost entirely. Therefore, when the molten solder cools in the metal sleeve it shrinks and it is possible that the solder may detach entirely from the glass fiber or leave capillaries which prevent achieving a hermetic seal. Furthermore, in the event of tensile stress, the fiber can slip out of the solder.
It is therefore an object of the invention to provide a hermetically sealed glass fiber bushing for feeding a glass fiber through a hole in a metal housing and for fixing the fiber in said hole. It is a further object to provide such a bushing which can be manufactured at reduced costs, offer excellent tightness, and be produced in the field, if necessary.
In accordance with the invention, these objects are uniquely achieved by using the characteristics of molten solder that created the problem, namely, shrinkage during cooling and surface tension. Excess solder is provided outside the hole to form a solder body surrounding and adhering to a bare unmetallized glass fiber. The solder body is also permanently connected with the outside of the metal housing and with at least part of the length of the wall of the hole.
In a second embodiment the hole has a metal sleeve soldered therein. The sleeve has an opening therethrough having a wall with nonsolderable coating over part of its length. Solder hermeticlly surrounds the bare, unmetallized fiber in the area of the nonsolderable part of the wall while in the solderable portion of the wall the solder firmly adheres to the wall.
In a third embodiment a metal sleeve is also used, the sleeve having a recess partly exposing the opening. In the area of the recess the solder hermetically surrounds the bare unmetallized fiber while in the area of the opening without the recess the solder adheres to the opening wall.
Numerous advantages are offered by the invention. Firstly, it is no longer necessary to metallize the glass fiber. Secondly, the complex washing steps hitherto required can be eliminated. Thirdly, the bushing has a high, reliable degree of tightness which can be achieved even if the bushings are produced in the field. Further advantages of the invention will become apparent as this description proceeds.